1. Field of the Invention
The present invention relates to a method and an apparatus for adaptively canceling an interference signal, and particularly relates to a method and an apparatus capable of canceling wide-band interference signals of any D/U ratio (a desired to undesired signal ratio) ranging from the positive to the negative, adapted for a digital transmission system by means of space-diversity reception.
2. Description of the Related Art
In radio relay systems, for example, for mobile telephone communication and digital microwave communication, to which PSK, QAM, etc., are applied, FM interference or jamming signals which possibly come from an adjacent analog channel are, in most cases, regarded as narrow band signals. However, interference signals which are transmitted from a digital transmission line are generally wide-band interference signals. This latter tendency is particularly prominent in high-speed digital transmission.
While cancellation of the narrow-band interference signals can be effected rather easily by employing a conventional elimination method using a linear or nonlinear filter, it is generally difficult to eliminate the wide-band interference signals.
It is known, however, that specifically when the power level of the interference signal predominates that of a desired signal (the D/U ratio is negative), cancellation of the interference signal can be advantageously effected by means of the power-inversion adaptive array concept, according to which interference signals, even when the signals are of a wide-band, can be canceled by combining in inverted phases two interference signals included in the arrival signals received by a two-branch space-diversity system. This technique is presented by R. T. Compton, Jr. in his paper entitled, "The Power-Inversion Adaptive Array: Concept and Performance," IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS, VOL. AES.multidot.15, NO. 6 NOVEMBER 1979.
FIG. 1 is a block diagram of an interference canceling apparatus having a two-branch space-diversity signal-reception system directed to working the Compton concept.
AGC (automatic gain control) amplifiers 308,309 assigned to first and second branches, respectively, amplify arrival signals T.sub.1, T.sub.2 to a normalized power level, producing first and second branch signals T.sub.3, T.sub.4, respectively. A correlator 306 receives both branch signal T.sub.3 and normalized output signal T.sub.N as a reference signal, generates a cross correlation coefficient between the two input signals and outputs the correlation coefficient as weight w.sub.1 or a tap coefficient, wherein the normalized signal T.sub.N is produced by diversity combining and normalization by AGC, as will be described below. Similarly, correlator 307 generates a cross correlation coefficient between branch signal T.sub.4 and normalized output signal T.sub.N, and outputs weight w.sub.2. We will now define "a correlation coefficient" in a complex expression. The correlation coefficient of any two complex quantities A and B is defined as a moving average of the product A*B over a sufficiently long time period, compared to a transmitted symbol duration of the desired signal S, so that a stable average value can be produced, and over a sufficiently short time period, compared to a fading duration on a transmission channel, so that the correlation coefficient can follow a channel fluctuation, and more precisely, so that the weights w.sub.1 and w.sub.2 provided from the correlater 306 and 307, respectively, can follow the transmission coefficients, h.sub.1, g.sub.1, h.sub.2 and g.sub.2, even when their amplitudes and phases vary randomly due to the fading. Multipliers 301, 302 multiply T.sub.3, T.sub.4 by weights w.sub.1, w.sub.2, respectively, thereby producing weighted branch signals T.sub.5, T.sub.6. An adder 303 adds the two weighted branch signals to effect diversity combining and produces sum signal T.sub.S. An AGC amplifier 305 amplifies signal T.sub.S to a normalized power level, thereby producing normalized signal T.sub.N. A subtractor 304 subtracts weighted branch signal T.sub.6 from weighted branch signal T.sub.5, thereby producing difference signal T.sub.D. A selector 310 receives normalized output signal T.sub.N and difference signal T.sub.D, and selectively outputs signal T.sub.N when the desired signal predominates the interference signal, and outputs signal T.sub.D when the interference signal is predominant over the desired signal. An equalizer 311 receives the output of the selector 310 and adaptively equalizes the received signal.
Before proceeding to the explanation of the operation of the apparatus shown in FIG. 1, several notations will be defined as follows:
m.sub.1, m.sub.2 automatic control gains of AGC amplifiers of the first and second branches, respectively, PA1 h.sub.1, h.sub.2 complex transmission coefficients for the desired signal representing the ratios of arrival desired signals D.sub.1, D.sub.2 at the first and second branches, respectively, to the transmitted desired signal S, i.e., D.sub.1 =h.sub.1 S and D.sub.2 =h.sub.2 S, wherein arrival desired signals D.sub.1, D.sub.2 refer to desired signal components in complex expression included in arrival signals T.sub.1, T.sub.2, respectively, PA1 g.sub.1, g.sub.2 complex transmission coefficients for the interference signal representing the ratios of arrival interference signals U.sub.1, U.sub.2 at the first and second branches, respectively, to the interference signal J, i.e., U.sub.1 =g.sub.1 J U.sub.2 =g.sub.2 J, wherein arrival interference signals U.sub.1, U.sub.2 refer to interference signal components in complex expression included in arrival signals T.sub.1, T.sub.2, respectively, and interference signal J refers to the signal as emitted from an interference signal source PA1 E[A*B] correlation coefficient of quantity A with respect to quantity B as defined above. PA1 amplifying a first arrival signal and a second arrival signal to a normalized power level by means of automatic gain control to produce a first branch signal and a second branch signal, respectively, the first arrival signal and the second arrival signal referring to the signals arriving at the first and second receiver means, respectively, represented in a complex expression, PA1 producing a first product and a second product, the first product being a product of amplitudes of a first interference signal and a second interference signal multiplied by a phase factor, the second product being a square of the amplitude of the second interference signal, the first and second interference signals being interference signals included in the first and second branch signals, respectively, and the phase factor being exp {i(.phi..sub.1 -.phi..sub.2)} with .phi..sub.1 and .phi..sub.2 denoting phase angles of the first and second interference signals, respectively, PA1 producing complex weights w.sub.1 and w.sub.2 so that the ratio w.sub.2 /w.sub.1 is equal to the ratio of the first product to the second product, PA1 multiplying the first and second branch signals by the complex weights w.sub.1 and w.sub.2, respectively, to produce a first weighted branch signal and a second weighted branch signal, respectively, PA1 performing subtraction of one of the two weighted branch signals from the other in order to compensate the first and second interference signals, and PA1 adaptively equalizing the result of the subtraction to produce a decision data signal. PA1 first and second amplifier means assigned to a first diversity branch and a second diversity branch, respectively, of the two-branch space-diversity signal-receiving system for amplifying a first arrival signal and a second arrival signal to a normalized power level by means of automatic gain control to produce a first branch signal and a second branch signal, respectively, wherein the first and second arrival signals refer to the signals arriving at the first and second amplifier means, respectively, PA1 weight-producing means for producing a first weight and a second weight by which the first and second branch signals, respectively, are to be multiplied, wherein the weight-producing means receives the first and second branch signals and a decision data signal and produces the first and second weights such that the ratio of the second weight to the first weight is equal to the ratio of a first product to a second product, the first product referring to a product of the amplitudes of a first interference signal and a second interference signal multiplied by a phase factor and the second product referring to a square of the amplitude of the second interference signal, the first and second interference signals being interference signals included in the first and second branch signals, respectively, and the phase factor being a factor corresponding to a complex factor exp {i(.phi..sub.1 -.phi..sub.2)} with .phi..sub.1 and .phi..sub.2 denoting phase angles of the first and second interference signals, respectively, PA1 multiplier means for multiplying at least one of the first and second branch signals by the corresponding weight to produce a weighted branch signal, PA1 subtractor means for performing subtraction between the weighted branch signals, and PA1 adaptive equalizer means for adaptively equalizing the output of the subtractor means to produce a decision data signal.
and
In operation, it is assumed for simplicity that normalized output signal T.sub.N is initially N.sub.0.sup.-1 (S+J), wherein N.sub.0 is a normalization constant to make the output power of AGC amplifier 305 equal to 1. Since branch signal T.sub.3 is EQU T.sub.3 =m.sub.1 (h.sub.1 S+g.sub.1 J), (1)
weight w.sub.1 generated by correlator 306 can be written ##EQU1## because a time average of a product of two uncorrelated quantities S*J or J*S gives zero. For simplicity, equation (2) may be rewritten EQU w.sub.1 =N.sub.0.sup.-1 (m.sub.1 h.sub.1 *.sigma..sup.2 +m.sub.1 g.sub.1 *.beta..sup.2), (3)
wherein positive real numbers .sigma., .beta. denote absolute values of S and J, respectively.
Thus, weighted branch signal T.sub.5 is ##EQU2## wherein R.sub.1 and C.sub.1 denote the terms of the first and second parenthesis multiplied by N.sub.0.sup.-1, R.sub.1 representing a phase-independent part with real coefficients of S and J, and C.sub.1 representing a phase-dependent part with complex coefficients. Similarly, weighted branch signal T.sub.6 is ##EQU3##
The sum of weighted branch signals is amplified by the AGC amplifier 305 to provide normalized output signal T.sub.N which is fed back to the correlators 306, 307 to cause the second loop-operation to be performed. In this way, recursive operations of the feed-back loop are iterated.
It should be noted that in the output of the adder 303, only the sum of the in-phase parts R.sub.1, R.sub.2 is effective. The reason for this is that since the terms in R.sub.1 and R.sub.2 have real coefficients, each of signals S and J included in R.sub.1 and R.sub.2 is combined in phase, with the result that the effect of the sum R.sub.1 +R.sub.2 is accumulated as the feed-back operation is iterated. Conversely, since the terms in C.sub.1 and C.sub.2 have phase-dependent (complex) coefficients, signals S and J included in C.sub.1 and C.sub.2 are individually added in random phase with the result that the maximal ratio combining effect of C.sub.1 +C.sub.2 is not attained as long as C.sub.1 and C.sub.2 have different phases from each other. For this reason, the phase-dependent part may generally be ignored from the output of the adder 303 after many iterations of feedback operation.
Based on the above argument, it follows that, for the first loop operation, ##EQU4## which causes the output of the AGC amplifier 305 to be EQU T.sub.N =N.sub.1.sup.-1 {(pS+qJ)+(phase-dependent term)} (7)
wherein EQU p=m.sub.1.sup.2 h.sub.1 h.sub.1 *.sigma..sup.2 +m.sub.2.sup.2 h.sub.2 .sub.2 *.sigma..sup.2, (9) EQU q=m.sub.1.sup.2 g.sub.1 g.sub.1 *.beta..sup.2 +m.sub.2.sup.2 g.sub.2 g.sub.2 *.beta..sup.2, (10)
and N.sub.1.sup.-1 denotes the normalization constant to normalize the power of signal T.sub.N.
Thus, normalized signal T.sub.N produced after n-time feed-back operations is EQU T.sub.N =N.sub.n.sup.-1 (p.sup.n S+q.sup.n J), (11)
wherein N.sub.n is again the normalization constant to normalize the power of signal T.sub.N. In equation (11), the ratio of the coefficient of S-term to that of J-term is EQU r=(p/q).sup.n ( 12)
Accordingly, if the powers of the arrival desired signals h.sub.1 h.sub.1 *.sigma..sup.2, h.sub.2 h.sub.2 *.sigma..sup.2 are predominant over those of the arrival interference signals g.sub.1 g.sub.1 *.beta..sup.2, g.sub.2 g.sub.2 *.beta..sup.2, respectively, then the inequality p&gt;q holds, as is known from equations (9) and (10). In this case, the desired signal component included in the normalized signal T.sub.N grows more predominant over the interference signal component as the number n of feedback increases. That is, normalized signal T.sub.N corresponds to the maximal ratio combined signal between the diversity branches when the power of the desired signal overcomes that of the intereference signal. The selector 310 connects the normalized output signal T.sub.N to the equalizer 311.
If the powers of the arrival interference signals surpass those of the arrival desired signals, then the interference signal component of T.sub.N becomes predominant, and difference signal T.sub.D is connected to the equalizer 311. In this case, the strong interference signal is effectively canceled in favor of the weak desired signal, as will be described with reference to FIG. 2.
FIG. 2 represents vector diagrams to intuitively illustrate the operation of the apparatus shown in FIG. 1. In FIG. 2, (a) through (e) represent the case in which the interference signal is stronger than the desired signal. This condition is represented by .beta.&gt;.sigma.. For simplicity, we assume that the terms including .sigma. are negligibly small.
As is easily derived from equation (11), weighted branch signal T.sub.5 is, after n-time feedback operations, EQU T.sub.5 =N.sub.n.sup.-1 (p.sup.n m.sub.1.sup.2 h.sub.1 h.sub.1 *.sigma..sup.2 S+q.sup.n m.sub.1.sup.2 g.sub.1 g.sub.1 *.beta..sup.2 J)+N.sub.n.sup.-1 (q.sup.n m.sub.1.sup.2 h.sub.1 g.sub.1 *.beta..sup.2 S+p.sup.n m.sub.1.sup.2 g.sub.1 h.sub.1 *.sigma..sup.2 J).(13)
Ignoring the terms including .sigma. gives ##EQU5## wherein S.sub.1 and J.sub.1 denote the desired and interference signal components included in branch signal T.sub.3, respectively, and S.sub.15 and J.sub.15 represent the desired and interference signal components, respectively, included in weighted branch signal T.sub.5.
As can be seen in equation (14), the coefficients of S.sub.1 and J.sub.1 are equal to each other and g.sub.1 * is a single complex number included in the coefficients. This means that the phase angle between S.sub.15 and S.sub.1 is equal to the phase angle between J.sub.15 and J.sub.1, and accordingly the angle between S.sub.1 and J.sub.1 is equal to that between S.sub.15 and J.sub.15 (cf. FIG. 2(a) and (c)). In addition, since the coefficient of J included in J.sub.15 (=N.sub.n.sup.-1 q.sup.n m.sub.1.sup.2 g.sub.1 g.sub.1 *.beta..sup.2) is a real number, the direction of J.sub.15 in the phase space should be parallel to the direction of J, i.e., J.sub.15 should have the same phase as J.
Similarly, the angle between the desired and interference components S.sub.2 and J.sub.2 in branch signal T.sub.4 is equal to the angle between the desired and interference components S.sub.26 and J.sub.26 (cf. FIG. 2(b), (d)), and J.sub.26 is parallel to J, wherein S.sub.26 and J.sub.26 refer to the desired and interference signal components, respectively, included in weighted branch signal T.sub.6. Thus, J.sub.15 and J.sub.26 are parallel to each other. As a result, the two interference components are combined in inverted phase by subtractor 304 (cf. FIG. 2(e)), whereby effective compensation is effected. On the other hand, the two desired signal components S.sub.15, S.sub.26 are not equidirectional to each other, i.e. they have uncorrelated phases. Accordingly, the absolute value of the resultant desired signal S.sub.D resulted from vector subtraction S.sub.15 -S.sub.26 is not always less than the absolute value of the component signal S.sub.15 or S.sub.26, as is shown in FIG. 2(e). In this way, the strong interference signal is effectively canceled in favor of the weak desired signal, thereby allowing the power ratio of the desired signal and interference signal to be inverted.
FIG. 2(f) through (j) shows the case in which the desired signal is stronger than the interference signal. In the figure, S.sub.1 and S.sub.2 denote the desired signal components included in branch signal T.sub.3 and T.sub.4, respectively, and J.sub.1 and J.sub.2 denote the interference signal components included in branch signal T.sub.3 and T.sub.4, respectively. In this case, we assume that .beta. is negligibly small. Based on this assumption, it follows that the phase angle between S.sub.1 and J.sub.1 is equal to the phase angle between S.sub.15 and J.sub.15 (cf. FIG. 2(f), (h)), and the phase angle between S.sub.2 and J.sub.2 is equal to the phase angle between S.sub.26 and J.sub.26 (cf. FIG. 2(g), (i)). It follows, furthermore, that both of the desired signal components S.sub.15 and S.sub.26 are parallel to (have the same phase as) the desired signal S, as J.sub.15 and J.sub.26 were parallel to J in the preceding case. For this reason, effective in-phase combination of S.sub.15 and S.sub.16 can be effected by the adder 303, as shown in FIG. 2(j). Interference signal components J.sub.15 and J.sub.26, on the other hand, direct to different orientations (have different phases). Consequently, the adder 303 effects combining of the two interference signal components J.sub.15, J.sub.26 in uncorrelated phases. In this way, the adder 303 acts in favor of the strong desired signal, although the interference signal is left uncanceled.
As described above, in the prior art interference-signal canceling apparatus, while the interference signal can be effectively canceled in the case of a negative D/U ratio, there remains the drawback that, in the case of a positive D/U ratio, effective combining of the desired signal causes the interference signal to be left uncanceled, and cancellation of the interference signal causes the desired signal also to be canceled.